Test a different C# keyword rename

Worked around warning in java jdk-15:
  warning: 'sealed' may become a restricted type name in a future release
  and may be unusable for type declarations or as the element type of an array

Adding sealed to the list of automatically renamed keywords in Java does
not seem prudent.

Examples/test-suite/keyword_rename.i

@@ -27,7 +27,7 @@ KW(synchronized, final)
 
 /* C# Keywords */
 KW(string, out)
-struct sealed {int i;};
+struct stackalloc {int i;};
 
 /* Go Keywords */
 KW(go, defer)